Semiconductor laser drive apparatus, optical write apparatus, imaging apparatus, and semiconductor laser drive method

ABSTRACT

A semiconductor laser drive apparatus and method are disclosed in order to obtain an optimal laser emission pulse that can reduce inherent light emission that may cause fogging and degradation of a photoconductor and also reduce turn on delay. Specifically, a light emission command signal is delayed at a delay unit based on a delay control signal so that a modulation signal is output. The light emission command signal and the delay signal are logically added to a command signal from an external source at a threshold signal generation unit so that a threshold ON signal is output. The modulation signal and the threshold ON signal respectively drive a modulation current switch and a threshold current switch so that a semiconductor laser drive current is generated. The threshold current is sampled according to a sample hold signal supplied from an external source and APC is performed on the sampled threshold current. In a differential quantum efficiency detection unit, an operation of determining differential quantum efficiency based on a current for obtaining a predetermined amount of light and a current for obtaining a portion of the predetermined amount of light is performed, and a light emission current is calculated. By adding the calculated light emission current and an arbitrary current that may be externally set, a modulation current for switching the semiconductor laser can be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser drive apparatususing a modulation signal that drives a semiconductor laser to emitlight, an optical write apparatus that implements such a semiconductorlaser drive apparatus, an imaging apparatus such as a laser printer, adigital copying machine, and a facsimile machine, that implements suchan optical write apparatus, and a semiconductor laser drive method usinga modulation signal for driving a semiconductor laser to emit light.

2. Description of the Related Art

Technologies relating to the present invention are disclosed, forexample, in Japanese Laid-Open Patent No.2001-88344 and JapaneseLaid-Open Patent No.2002-32140.

Japanese Laid-Open Patent No.2001-88344 discloses an imaging apparatussuch as a digital copying machine that is adapted to form an image on aphotoconductor drum by scanning the photoconductor drum with a laserbeam output from a semiconductor laser oscillator so as to constantlyobtain a stable optical output intensity regardless of a change in theenvironmental temperature and to thereby form an image with a stabledensity, in which apparatus light emission level stabilization controlis performed on a light emission level of the semiconductor laseroscillator during a non-image formation time and a light emission levelof the semiconductor laser oscillator during an image formation time sothat both light emission levels may be maintained at their respectivepredetermined values.

Japanese Laid-Open Patent No.2002-32140 discloses an imaging apparatushaving a cheap and small structure that is adapted to modulate asemiconductor laser using a modulation signal to realize high speedmodulation control of the semiconductor laser and acquire an opticalquenching ratio, and form an electrostatic latent image on aphotoconductor by scanning an optical beam from the semiconductor laser,the apparatus including a first current drive unit that supplies to thesemiconductor laser a first current that does not actually cause thesemiconductor laser to emit light but causes it to be in a state thatallows high speed modulation, which first current is supplied at a firsttiming based on the modulation signal; and a second current drive unitthat supplies to the semiconductor laser a second current that is turnedon/off according to the modulation signal, which second current issupplied at a second timing that is later than the first timing based onthe modulation signal; wherein the semiconductor laser is arranged toemit light by the combined current of the first current and secondcurrent.

In a semiconductor laser (LD), a threshold current (Ith) and anoperation current (Iop) change depending on the temperature or theelapsed time, and thereby, a monitor photodiode (PD) that is implementedin the semiconductor laser monitors the amount of light being output andcontrols the current being supplied to the semiconductor laser so thatthe amount of light output may remain fixed. Accordingly, in aconventional optical write apparatus that performs on/off control of asemiconductor laser according to image data being input and scans a beamto write an image, a semiconductor laser drive circuit may use thefollowing control methods:

{circle around (1)} supplying the operation current (Iop) with which apredetermined amount of light can be obtained during light-on time, andnot supplying the operation current during light-off time.

{circle around (2)} reducing a turn on delay time by supplying a biascurrent (Ib) during light-off time in order to increase the switchingspeed.

{circle around (3)} setting a modulation current to a fixed value inorder to reduce variations in the turn on delay, and changing the biascurrent in accordance with a change in the threshold current.

FIG. 1 is a graph illustrating a relation between an applied current ina semiconductor laser drive circuit that performs control according tothe above control method {circle around (1)} and an amount of light.FIG. 2 is a circuit diagram showing a circuit configuration of thissemiconductor laser drive circuit. According to this arrangement, thesemiconductor laser drive circuit includes a semiconductor laser LD, aswitch 1, a current source 2, a sample hold circuit 3, an amplifier 4,and a photodiode PD. The switch 1 is operated based on a modulationsignal and performs on/off control of a modulation current applied tothe semiconductor laser. The current source 2 supplies a drive currentto the semiconductor laser LD according to a voltage set by the samplehold circuit 3. The photodiode PD feeds back a light emission quantityof the semiconductor laser LD as a feedback signal to the amplifier 4.According to a sampling signal from an external source, the sample holdcircuit 3 samples an output from the difference amplifier 4 thatreceives the feedback signal from the photodiode PD and a light emissioncontrol voltage, and performs APC operations. Then, the APC-producedvoltage is applied to the current source 2.

In such an arrangement, it is known that, owing to the requiredexcitation time for the semiconductor laser LD, it takes time ns for thesemiconductor laser LD to emit light from the time a current issupplied. This is referred to as turn on delay (refer to FIG. 3 andUnderstanding Fundamentals and Applications of Semiconductor Lasers;Hirata, Shoji; CQ Publishing Co., Ltd.; 2001). Also, the amplitude ofthe switching current is relatively large, and it is therefore difficultto increase the switching speed of the switch element (e.g.,transistor). Thus, the laser emission rise time and fall time tend to belong.

FIGS. 4 and 5 respectively illustrate a current-light characteristic anda circuit configuration of a semiconductor laser drive apparatus thatperforms control according to the control method {circle around (2)}. Inthis method, during light-off time, a bias current with a fixed value issupplied. Thus, a bias current source 5 corresponding to a currentsource for the bias current is implemented parallel to the switch 1 andthe current source 2 for the modulation current as is shown in FIG. 5.Other components of this semiconductor laser drive apparatus areidentical to those shown in FIG. 2.

According to this method, the bias current needs to be arranged so thatit does not exceed the threshold current Ith even when the thresholdcurrent (Ith) changes due to environmental temperature change andvariations in the elements, and thus, the bias current cannot take alarge value as is shown in FIG. 4. Therefore, the effects of supplyingthis bias current are not very adequate, and differences in the turn ondelay time may occur since switching is performed without taking intoconsideration the potential differences in the actual thresholdcurrents. Also, in FIG. 4, the threshold current Ith is represented bythe intersecting point between the horizontal axis indicating no changein the amount of light and an extension of the line of the current-lightcharacteristic curve at 25° C. with the greater inclination (slope)indicating a greater change rate of the amount of light with respect tothe modulation current amount. This threshold current is arranged to begreater than the fixed bias current. The slope of the line intersectingthe horizontal axis may also be referred to as the differential quantumefficiency, and this differential quantum efficiency is represented byη.

FIGS. 6 and 7 show a current-light characteristic and a circuitconfiguration of a semiconductor laser drive circuit that performscontrol according to the control method {circle around (3)}. Accordingto this method, the bias current source 5 as is shown in FIG. 5 isarranged to supply a threshold current as the bias current (see FIG. 7).Other components of this semiconductor laser drive circuit are identicalto those shown in FIG. 5. It is noted that this arrangement correspondsto the first prior art document described above (Japanese Laid-OpenPatent No.2001-88344). In this arrangement, the bias current iscontrolled to correspond to the threshold current so that turn on delaycan be minimized and optimal light emission can be realized.

However, since the threshold current is constantly being suppliedaccording to this arrangement, light may inherently be emitted even whenlight emission is unnecessary (i.e., during light-off period). Althoughthe inherent light emission may not amount to much, the light is stillirradiated onto the photoconductor which may lead to fogging anddegradation of the photoconductor drum. Also, as can be appreciated fromFIGS. 1, 4, and 6, the differential quantum efficiency (slope) tends todecrease with the increase in temperature; thus, when the modulationcurrent detected at a low temperature is used as the fixed modulationcurrent, at high temperature, the bias current may exceed the actualthreshold current thereby increasing the offset light emission.

SUMMARY OF THE INVENTION

The present invention has been conceived in response to the problems ofthe related art, and its object is to provide a semiconductor laserdrive apparatus and a semiconductor laser drive method for obtaining anoptimal laser emission pulse that is able to reduce inherent lightemission as well as turn on delay so that the semiconductor laser canemit light without delay and fogging and degradation of thephotoconductor can be prevented.

Another object of the present invention is to provide an optical writeapparatus that implements a semiconductor laser drive apparatus and animaging apparatus that implements such an optical write apparatus, thesemiconductor laser drive apparatus being adapted to obtain an optimallaser emission pulse that is able to reduce inherent light emission aswell as turn on delay so that the semiconductor laser can emit lightwithout delay and fogging and degradation of the photoconductor can beprevented.

Specifically, a semiconductor laser drive apparatus according to anembodiment of the present invention modulates a semiconductor laseraccording to a modulation signal and induces the semiconductor laser toemit light, the apparatus including:

a control unit that is adapted to supply a fixed bias current during alight emission off time, and start supplying a predetermined currentthat is less than a light emission threshold current right before alight emission time.

A semiconductor laser drive apparatus according to another embodiment ofthe present invention modulates a semiconductor laser according to amodulation signal and induces the semiconductor laser to emit light, theapparatus including:

a control unit that is adapted to refrain from supplying a currentduring a light emission off time, and start supplying a predeterminedcurrent that is less than a light emission threshold current rightbefore a light emission time. .

A semiconductor laser drive apparatus according to another embodiment ofthe present invention modulates a semiconductor laser according to amodulation signal and induces the semiconductor laser to emit light, theapparatus including:

a control unit that is adapted to supply a predetermined current that isless than a light emission threshold current when a light emissioncommand signal is received, and supply a modulation current after apredetermined time period passes from the time the predetermined currentis supplied.

Further, in the semiconductor laser drive apparatus of the presentinvention, the predetermined current may correspond to a current that isclose to the light emission threshold current.

In another embodiment, the predetermined current may correspond to a sumof the bias current and a current obtained from sampling a lightemission state of the semiconductor laser.

In another embodiment, the control unit may be adapted to determine adifferential quantum efficiency at least in one of a case where power isturned on and a case where a job is to be started.

In another embodiment, the control unit may be adapted to determine adifferential quantum efficiency at predetermined time intervals.

In a further embodiment, the control unit may be adapted to determinethe differential quantum efficiency based on a current for obtaining apredetermined amount of light, and a current for obtaining a prescribedportion of the predetermined amount of light.

In another embodiment, the control unit may include a function forsetting a difference between the light emission threshold current andthe predetermined current that is less than the light emission thresholdcurrent and is supplied right before the light emission time.

In a further embodiment, the difference may be set by means of anexternal terminal.

In another embodiment, the difference may be set to a value that isgreater than or equal to a difference between a light emission currentat a time of initialization and a light emission current at a time whenthe environmental temperature is increased from the time ofinitialization.

In another embodiment, a supply time for supplying the predeterminedcurrent that is less than the light emission threshold current may bearbitrarily set.

In another embodiment, a signal indicating a supply time for supplyingthe predetermined current that is less than the light emission thresholdcurrent may be input, which signal is independent from a signalindicating a drive time for driving the semiconductor laser to emit apredetermined amount of light.

In another embodiment, the control unit may include:

a modulation current source that is adapted to supply a modulationcurrent to the semiconductor laser based on a switching operationrealized by a modulation signal;

a bias current source that is implemented parallel to the modulationcurrent source and is adapted to supply the bias current having a fixedvalue; and

a control current source that is implemented parallel to the modulationcurrent source and is adapted to supply a control current that is set bya sample hold circuit based on a switching operation realized by athreshold ON signal.

An optical write apparatus according to the present invention includes asemiconductor laser drive apparatus of the present invention, and awrite unit for realizing optical writing on an image sustaining elementby scanning a laser beam emitted from a semiconductor laser that isdriven by the semiconductor drive apparatus using a polygon mirror.

In a further embodiment, the optical write apparatus of the presentinvention may include a temperature detection unit for detecting thetemperature of the semiconductor laser or a location in the vicinity ofthe semiconductor laser, and an initialization unit for performinginitialization of the semiconductor laser drive apparatus based on thetemperature detected by the temperature detection unit.

An imaging apparatus of the present invention includes:

an optical write apparatus of the present invention;

an image developing unit that is adapted to develop an image written onthe image sustaining element by means of the optical writing apparatus;and

a recording unit that is adapted to record the image developed by theimage developing unit on a recording medium.

In a further embodiment, the imaging apparatus of the present inventionmay include an input apparatus that is adapted to input imageinformation based on which an image is recorded on the recording medium.

A semiconductor laser drive method of the present invention formodulating a semiconductor laser based on a modulation signal andinducing the semiconductor laser to emit light includes:

supplying a fixed bias current during a light emission off time; and

starting to supply a predetermined current that is less than a lightemission threshold current right before a light emission time.

A semiconductor laser drive method according to another embodiment ofthe present invention for modulating a semiconductor laser based on amodulation signal and inducing the semiconductor laser to emit lightincludes:

refraining from supplying a current during a light emission off time;and

starting to supply a predetermined current that is less than a lightemission threshold current right before a light emission time.

A semiconductor laser drive method according to another embodiment ofthe present invention for modulating a semiconductor laser based on amodulation signal and inducing the semiconductor laser to emit lightincludes:

starting to supply a predetermined current that is less than a lightemission threshold current when a light emission command signal isreceived; and

starting to supply a modulation current after a predetermined timeperiod passes from the time the predetermined current starts beingsupplied.

In a further embodiment, the semiconductor laser drive method of thepresent invention may include setting a supply time for supplying thepredetermined current that is less than the light emission thresholdcurrent to an arbitrary time.

In another embodiment, the semiconductor laser drive method of thepresent invention may include inputting a signal indicating a supplytime for supplying the predetermined current that is less than the lightemission threshold current, which signal is independent from a signalindicating a drive time for driving the semiconductor laser to emit apredetermined amount of light.

According to the semiconductor laser drive apparatus and method of thepresent invention, a current is not supplied or a fixed bias current issupplied during a light emission off time, and a predetermined currentthat is close to but less than a light emission threshold current startsbeing supplied right before a light emission time so that inherent lightemission may be prevented when light emission is undesired. In otherwords, the predetermined current starts being supplied when a lightemission command signal is received, and a modulation current forinducing the light emission of the semiconductor laser starts beingsupplied after a predetermined time period passes from the time thepredetermined current starts being supplied. In this way, the lightemission time is delayed from the light emission signal supply time bythe predetermined time period during which time period the predeterminedcurrent is supplied so that the semiconductor laser may be modulatedwithout turn on delay. Thus, a laser emission pulse that can reduceinherent light emission as well as turn on delay can be obtained.

Further, since the light emission threshold current may be determinedwhen the power is tuned on or when a job is to be started, undesired orunnecessary light emission for the determination of a differentialquantum efficiency may be minimized, and for example, when thisembodiment is applied to an imaging apparatus, unnecessary or undesiredtoner image may be prevented from being formed on the photoconductor andthe degradation of the photoconductor may be prevented.

Also, since the differential quantum efficiency may be determined atpredetermined intervals, and the predetermined current that is less thanthe light emission threshold current may be controlled based on thedetermined differential quantum efficiency, inherent light emission canbe prevented from being unnecessarily large even without implementing atemperature detection unit.

Also, since the differential quantum efficiency may be obtained based ona current for obtaining a predetermined amount of light and a currentfor obtaining a portion of the predetermined amount of light, theoperation of determining the light emission threshold current may berealized in a practical manner.

Also, by allowing the difference between the light emission thresholdcurrent and the predetermined current that is less than the lightemission threshold current to be set arbitrarily, inherent lightemission due to the predetermined current that is less than the lightemission threshold current may be reduced, and inherent light emissioncan be prevented from being unnecessarily large even when a changeoccurs in the difference between the threshold current and the drivecurrent due to a change in the differential quantum efficiency, whichchanges according to environmental temperature change.

Further, since the difference may be arbitrarily set by an externalterminal, this difference may be set according to the characteristics ofthe semiconductor laser.

Also, when the difference is set to a value that is greater than orequal to a difference between a current at an initialization time and acurrent at a higher temperature, for example, to several mA, both theturn on delay at a light emission time and inherent light emission dueto the predetermined current may be prevented.

Also, when a supply time for supplying the predetermined current may bearbitrarily set, both the turn on delay at a light emission time andinherent light emission due to the predetermined current may beprevented.

When a signal indicating a supply time for supplying the predeterminedcurrent is input independently from the signal indicating the drive timeof the semiconductor laser, the predetermined current may be suppliedwithout other restrictions, and both the turn on delay at a lightemission time and inherent light emission due to the predeterminedcurrent may be prevented. Also, even when the semiconductor laser driveapparatus is realized by an analog device so that implementing alarge-scale logic circuit would be costly, by implementing such logiccircuit in a digital IC preceding the analog device, an inexpensivesemiconductor laser drive apparatus may be realized.

Also, by supplying a control current that is set by a sample holdcircuit based on a switching operation realized by a threshold ONsignal, inherent light emission time due to the predetermined currentmay be reduced.

Also, by supplying a temperature detection unit and an initializationunit, the temperature of the semiconductor laser (environmentaltemperature) may be monitored, and the differential quantum efficiencymay be determined according to temperature increase to control thepredetermined current, so that the amount of inherent light emission maybe prevented from becoming unnecessarily large even when temperaturechange occurs within the apparatus.

It is noted that in the following descriptions of the preferredembodiments of the present invention, Ibi corresponds to the biascurrent, ASIC 30 represents the control unit as a whole, Ith correspondsto the light emission threshold current, Ibi+Ish corresponds to thepredetermined current that is equivalent to the sum of the bias currentand the current obtained by sampling a light emission state of thesemiconductor laser, Isub corresponds to the function for setting thedifference between the predetermined current and the light emissionthreshold current, Isub setting terminal corresponds to the externalterminal, the threshold ON signal corresponds to setting the supply timeof the predetermined current and inputting a signal indicating thesupply time for supplying the predetermined current (setting isperformed at a preceding IC), the current source 2 corresponds to themodulation current source, the current source 5 corresponds to the biascurrent source, the current source 6 corresponds to the control currentsource, the temperature sensor 15 corresponds to the temperaturedetection unit, and the LD driver 30 corresponds to the initializationunit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a characteristic relation between an appliedcurrent and an amount of light emission in an application of asemiconductor laser drive circuit that controls a semiconductor laser bysupplying a current for obtaining a predetermined amount of light duringa light-on period, and supplying no current during a light-off periodaccording to the related art;

FIG. 2 is a block diagram showing a circuit configuration of thesemiconductor laser drive circuit of the related art that realizes thecharacteristic shown in FIG. 1;

FIG. 3 is a timing diagram showing waveforms representing a turn ondelay according to the characteristic of FIG. 1;

FIG. 4 is a graph showing a characteristic in an application of aconventional semiconductor laser drive circuit that performs control bysupplying a bias current that has a fixed value during the light-offperiod to counter the turn on delay;

FIG. 5 is a block diagram showing a circuit configuration of thesemiconductor laser drive circuit of the related art that realizes thecharacteristic shown in FIG. 4;

FIG. 6 is a graph showing a characteristic in an application of asemiconductor laser drive circuit that performs control by maintaining amodulation current at a fixed value in order to reduce differences inthe turn on delay, and supplying a bias current that changes accordingto a change in a threshold current according to the related art;

FIG. 7 is a block diagram showing a circuit configuration of thesemiconductor laser drive circuit of the related art that realizes thecharacteristic shown in FIG. 6;

FIG. 8 is a schematic diagram showing a mechanical configuration of animaging apparatus according to an embodiment of the present invention;

FIG. 9 is a block diagram showing a configuration of a write unitaccording to an embodiment of the present invention;

FIG. 10 is a graph showing a current-light emission characteristic (ILcurve) of a semiconductor laser according to an embodiment of thepresent invention;

FIG. 11 is a timing diagram showing waveforms of signals used in acircuit configuration of FIG. 12 to realize the characteristic of FIG.10;

FIG. 12 is a block diagram showing a circuit configuration of asemiconductor laser drive apparatus according to an embodiment of thepresent invention;

FIG. 13 is a block diagram showing a circuit configuration in a casewhere the semiconductor laser drive apparatus circuit of FIG. 12 isconfigured by a one-chip ASIC;

FIG. 14 is a graph illustrating a method of determining a light emissioncurrent in the circuit of FIG. 13;

FIG. 15 is a block diagram showing a relation between the ASIC realizingthe semiconductor laser drive circuit (LD driver) of the presentembodiment, a CPU that administers control over the entire semiconductorlaser drive apparatus, and an image input apparatus;

FIG. 16 is a timing chart according to an embodiment of the presentinvention illustrating timings of performing on/off control according tolight-on and light-off signals;

FIG. 17 is another timing chart according to an embodiment of thepresent invention illustrating timings of performing on/off controlaccording to light-on and light-off signals;

FIG. 18 is a graph showing a characteristic in a case of correcting thethreshold current Ith when temperature change occurs upon completion ofan initialization process according to an embodiment of the presentinvention;

FIG. 19 is a graph showing a characteristic in a case where thetemperature of the semiconductor laser increases after initialization,and the differential quantum efficiency of the semiconductor laserdecreases with the increase in temperature, so that a current suppliedduring an Ith generation period exceeds the actual threshold currentIth, resulting in the semiconductor laser emitting light, according toan embodiment of the present invention;

FIG. 20 is a diagram showing a characteristic in a case where a DACcurrent corresponding to a sum of the determined light emission currentand an arbitrary current is used in order to prevent the semiconductorlaser from emitting light during the Ith generation period when thetemperature increases after the initialization;

FIG. 21 is a timing chart showing the output timings of signals torealize the characteristic of FIG. 20; and

FIG. 22 is another timing chart showing the output timings of signals ina case where the timing of the Ith generation period may be arbitrarilycontrolled by an external terminal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the present invention aredescribed with reference to the accompanying drawings.

FIG. 8 is a schematic diagram illustrating a mechanical configuration ofan imaging apparatus according to an embodiment of the presentinvention. FIG. 9 is a diagram illustrating a configuration of a writeunit. The imaging apparatus shown in FIG. 8 is roughly made up of anautomatic document feeder (ADF) 200 and an imaging apparatus main body100. In the ADF 200, a stack of documents 210 may be placed on adocument holder 220 with the image side up. A document detection sensor209 may detect the documents 210 set in the document holder 220, andwhen a start (print) key of an operation unit (not shown) is pressed,the document sheets 210 may be fed to the apparatus starting with thedocument sheet at the top of the stack. When the front edge of adocument sheet reaches the position where a resist sensor 205 is set,the paper feeding operation temporarily halts. Then, the operationrestarts and the document that has reached the resist sensor 205 is readby a read unit 150 at an. ADF document read position 110 of the imagingapparatus main body 100. In the case where only one side of the documentis read, the document is discharged to a document discharge tray 207. Inthe case where both sides of the document are read, the document isflipped at the document discharge tray 207 and re-fed to the apparatusthrough a document separator 206 so that the other side of the documentis read. Also, documents such as pages of a book are placed on a contactglass 108 so that the read unit 150 may read the document.

The imaging apparatus 100 has a first paper feed tray 111, a secondpaper feed tray 112, and a third paper feed tray 113, respectivelyimplementing a first paper feed apparatus 114, a second paper feedapparatus 115, and a third paper feed apparatus 116. Sheets of transferpaper are stocked in each of the paper feed trays, and a transfer papersheet stocked in any one of the paper feed trays may be fed to theapparatus through its corresponding paper feed apparatus. The sheet iscarried by a common vertical carrier unit 117 through a conveyance route26 to a resist roller 117 a, and is further carried from the resistroller 117 a toward a photoconductor 118 so as to be in contact with thephotoconductor 118. The image data read by the read unit 150 are writtenon the photoconductor 118 by a laser beam from a write unit 157, and atoner image is formed on the photoconductor 118 by a developing unit127. The transfer paper sheet is carried by a conveyor belt 119 at aspeed matching the rotational speed of the photoconductor 118 so thatthe toner image formed on the photoconductor 118 is transferred onto thetransfer paper sheet. Then, the image transferred onto the transferpaper sheet is fixed at a fixing unit 120, after which the transferpaper is discharged to a discharge tray 121.

In the case of forming an image on both sides of the transfer papersheet, instead of the sheet being guided toward the discharge tray 121from the fixing unit 120, the sheet is carried to a duplex side conveyorpath 132 by separators 131 used for changing paths. Then, the sheetcarried by the duplex side conveyor path 132 is re-fed to the verticalcarrier unit 117 so that a toner image formed on the photoconductor 118can be transferred onto the other side of the transfer paper sheet.After the second toner image transfer operation, the transferred imageis fixed at the fixing unit 120 and the transfer paper sheet is guidedtoward the discharge tray 121.

The read unit 150 is made up of the contact glass 108 on which adocument can be placed and an optical scanning system. The opticalscanning system may include an exposure lamp 151, a first mirror 152, asecond mirror 155, a third mirror 156, a lens 153, and a CCD imagesensor 154, for example. The exposure lamp 151 and the first mirror 152may be fixed on a first carriage (not shown) and the second mirror 155and the third mirror 156 may be fixed on a second carriage (not shown).

In reading a document image from the contact glass 108, the firstcarriage and the second carriage may be mechanically moved at relativespeeds of 2 to 1. This optical scanning system may be driven by ascanner drive motor (not shown). On the other hand, in reading adocument image of a document that is supplied from the ADF 200, theexposure lamp 151 and the first mirror 152 mounted on the firstcarriage, and the second mirror 155 and the third mirror 156 mounted onthe second carriage are fixed to their positions as shown in FIG. 8. Thedocument image is read by a CCD image sensor 154, and the read imagedata are converted into an electrical signal to be processed.

The write unit 157 is made up of a laser output unit including a polygonmirror 158, an imaging lens 159, and a mirror 160. The laser output unitimplements a semiconductor laser (LD) corresponding to a laser beamsource and the polygon mirror 158 that rotates at a fixed highrotational speed by means of a motor. A laser beam that is emitted fromthe semiconductor laser LD is deflected at the polygon mirror 158, afterwhich it passes through the imaging lens 159 to be reflected at themirror 160 so that a latent image is condensed onto the surface of thephotoconductor 118. The deflected laser beam is scanned in a mainscanning direction that is perpendicular to the rotating direction ofthe photoconductor 118, and realizes recording in line units of an imagesignal output from an output data selector of an image processing unit(not shown). By repeating the main scanning operation in predeterminedcycles according to the rotational speed of the photoconductor 118 andthe recording density, an image (electrostatic latent image) may beformed on the surface of the photoconductor 118.

The laser beam output from the write unit 157 is irradiated onto thephotoconductor 118 of an image forming system. As is shown in FIG. 9, abeam sensor 130 that generates a main scanning synchronization signal isimplemented close to one end of the photoconductor 118 at a positionwhere the laser beam is irradiated. Based on the main scanningsynchronization signal, a start timing of image recording in the mainscanning direction is controlled, and a control signal for inputting andoutputting an image signal is generated. It is noted that FIG. 9 alsoshows an ASIC 30, a CPU 31, and an image input apparatus 32, which aredescribed below.

In the following, an operation according to an embodiment of the presentinvention is described by referring to FIG. 10 showing a current-lightquantity characteristic (IL curve) of a semiconductor laser LD accordingto the present embodiment, FIG. 11 showing waveforms of various signalsused in the present embodiment, and FIG. 12 showing a circuitconfiguration of a semiconductor laser drive apparatus according to thepresent embodiment.

As is shown in the circuit diagram of FIG. 12, according to the presentembodiment, a switch 7 and a current source 6 supplying a currentindicated as (threshold current-Isub) are serially connected, and theserially connected switch 7 and current source 6 are implementedparallel to the bias current source 5 shown in FIG. 5. Other componentsof the circuit configuration of FIG. 12 are identical to those of FIG.5. The current being supplied to the bias current source 5 has a fixedvalue, and the light emission of the semiconductor laser LD (offsetlight emission) by this current needs to be arranged so that fogging anddegradation of the photoconductor 118 are not caused. For example, ifthe current is set to be in the order of 1 mA, the offset light emissionwill hardly occur but the switching speed can be increased since aforward voltage is being applied to the semiconductor laser. If such aneffect is not desired, the current does not have to be supplied, andrefraining from supplying this current does not hinder the effects ofthe present invention. Also, as is explained below with reference toFIG. 20, Isub corresponds to a current having an arbitrary value(current value) that is greater than or equal to a value correspondingto the difference between a light emission current IηN at aninitialization time and a light emission current IηH at a transitiontime to a higher environmental temperature, this current value being setto a value that will not allow the amount of light in the offset lightemission to reach the amount that enables writing on the photoconductor118 even when the temperature is raised. The current Isub is referred toas “arbitrary current” hereinafter.

In FIGS. 10 and 11, a current that is close to the threshold current Ithbut is less than the threshold current Ith (threshold currentIth-arbitrary current Isub) is supplied right before the modulationcurrent is supplied. The modulation current is then supplied in additionto this current so that the semiconductor laser LD emits a predeterminedamount of light. Since the threshold current Ith of the semiconductorlaser LD changes depending on the environmental temperature, APC (autopower control) is performed to obtain the current (threshold currentIth-arbitrary current Isub) as a control current. APC corresponds to anoperation of maintaining the amount of light being emitted at a fixedvalue during light emission by controlling the current being supplied tothe semiconductor laser LD during the light emission period so that anoutput current of a photodiode PD implemented in the semiconductor laserLD is maintained at a fixed value. As is known to persons skilled in theart, APC may be performed, for example, in between writing lines forimage writing, in between changing sheets of paper, or in some cases,during a light-on period in which an image is being written.

In FIG. 11, an example is shown in which a current obtained bysubtracting the arbitrary current Isub from the threshold current valueIth (Ith-Isub) is supplied 1˜10 ns before the modulation current issupplied. With this control current, turn on delay and overshoot of theoptical waveform of the semiconductor laser LD may be prevented, therebyimproving the optical waveform. As is described above, the differentialquantum efficiency (slope) tends to decrease with the increase inenvironmental temperature. Thus, in the prior art, when a modulationcurrent detected at a low temperature is used as a fixed modulationcurrent, at high temperature, the bias current may exceed the actualthreshold current thereby increasing the offset light emission. Bysubtracting the arbitrary current Isub from the threshold current Ith asin the present embodiment, the above described problem can be solved.The IL curve of FIG. 10 illustrates how this is realized. In thisexample, the modulation current is arranged to correspond to a currentobtained by adding the arbitrary current Isub to the difference betweenthe threshold current Ith detected at 25° C. and the operation currentIop for obtaining a predetermined amount of light P0 at 25° C. (lightemission current Iη). According to the present example, even when thetemperature of the semiconductor laser LD is raised to 60° C. so thatthe light emission current Iη is slightly increased, the current beingsupplied to the semiconductor laser LD during light-off time will notexceed the corresponding threshold current Ith for 60° C. since thearbitrary current Isub is used as a margin. In this way, the offsetlight emission can be reduced.

As is described above, the arbitrary current may have an arbitrary valuethat is greater than or equal to the difference between the lightemission current IηN at the time of initialization and the lightemission current IηH when the temperature is raised. When the arbitrarycurrent Isub is set to have a large value, a wide margin can be providedfor the offset light emission, but this in turn degrades the effects ofreducing influences from turn on delay and overshoot of the opticalwaveform. Thus, the optimal current value for the arbitrary current Isubmay be within a range of 1 mA to several mA. It is noted that since theoptimal current value for the arbitrary current Isub varies depending onthe semiconductor laser and the optical write apparatus, the value ofthe arbitrary current Isub may be set at an external circuit of an IC.The setting of the arbitrary current is performed by an Isub settingterminal shown in FIG. 13, and the details of this operation aredescribed below.

In considering a practical case of using the control current (thresholdcurrent Ith-arbitrary current Isub) in a laser printer or a digitalcopying machine, for example, as long as the time at which the controlcurrent starts being supplied is just a short while of about 1˜10 nsbefore the time the modulation current is supplied, the image beingformed is not likely to be influenced by the control current. That is,although offset light may be generated to a certain extent, the lightemission from the control current amounts to forming no more than onedot on the photoconductor 118. Also, depending on the characteristics ofthe semiconductor laser LD, a bias current of a certain level may haveto be supplied in order to achieve a predetermined power level.Otherwise, a significant amount of time may be required to reach thepredetermined power level. Similarly, advantageous effects may beobtained from supplying a bias current of a certain level in a casewhere the semiconductor laser LD has poor droop characteristics.Accordingly, in the present embodiment, a supply time at which thecontrol current (threshold current Ith-arbitrary current Isub) issupplied may be freely set.

Alternatively, instead of setting the supply time of the control current(threshold current Ith-arbitrary current Isub), an external pin may beprovided for inputting a signal generated at a digital ASIC. Byimplementing the external pin, the control current (threshold currentIth-arbitrary current Isub) may easily be arranged to be supplied at alltimes, for example.

As is described above, according to the present embodiment, themodulation current has a fixed value even when APC is performed. On theother hand, the difference between the threshold current Ith and theoperation current for obtaining the predetermined amount of light P0(light emission current Iη) may vary depending on the semiconductorlaser LD and the amount of light. Thus, there is a need to determine thelight emission current Iη. Determining the light emission current Iη(initialization) involves lighting the semiconductor laser LD that isunrelated to a scanning position, and results in unnecessarilyirradiating the laser onto the photoconductor. Therefore, this operationmay be performed when the power is switched on or when a job is to bestarted.

In the following, the method of determining the light emission currentIη is described with reference to FIG. 14.

In this method, it is presumed that the light emission current Iηexceeding the threshold value Ith can approximately be represented by astraight line. First, the LD current is controlled so that thepredetermined amount of light P0 can be obtained, and the correspondingLD current value is stored as Iopl. Then, the LD current is controlledto obtained an amount of light that is ½ of the predetermined amount oflight P0 (P0/2), and the corresponding current value is stored as Iop2.Since Iop1-Iop2 corresponds to ½ of the light emission current Iη, thelight emission current Iη may be obtained by multiplying the valueIop1-Iop2 by 2. It is noted that in the present example, the lightemission current Iη is determined using ½ of the predetermined amount oflight P0; however, the determination may in fact be realized using anydenominator.

FIG. 13 shows a circuit configuration in a case where the semiconductorlaser drive circuit shown in FIG. 12 is made up of a one-chip ASIC. Itis noted that component parts identical to those shown in FIG. 12 aregiven the same referential notations and their descriptions are omitted.

The ASIC 30 includes a switch 1, current sources 2, 5, and 6, a samplehold circuit 3, an amplifier 4, a switch 7, a timing generation unit 8,a differential quantum efficiency detection unit 9, a digital/analogconversion unit (D/A or DAC) 10, an adder 11, a delay unit 12, and athreshold signal generation unit 13.

A light emission command signal is input to the delay unit 12, and thelight emission command signal is delayed at the delay unit 12 based on adelay control signal to be output as a modulation signal. Also, thelight emission command signal and the delay control signal are logicallyadded (OR) with a command signal from an external source (i.e., thethreshold ON signal and sample hold signal in this example) to output athreshold ON signal. The modulation signal and the threshold ON signalrespectively drive the modulation current switch 1 and the thresholdcurrent switch 7 so that an LD drive current as shown in FIG. 16 may begenerated. Also, the threshold current Ith is sampled by a sample holdsignal from an external source to perform APC. The sample hold signalmay be input for every line or every few lines, or in between pages inwhich case the light emission command signal is turned on and the samplehold signal is input while the semiconductor laser LD emits light.

At the differential quantum efficiency detection unit 9, an operation isperformed for obtaining the differential quantum efficiency η from thecurrent for obtaining a predetermined amount of light emission and acurrent for obtaining a certain portion of the predetermined amount oflight emission based on an initialization command signal from anexternal source and a signal output from the timing generation unit 8 soas to determine the emission current Iη. Then, an arbitrary current Isubthat may be externally set is added to the emission current Iη togenerate the modulation current for switching the semiconductor laserLD. The bias current source 5 corresponds to a circuit that constantlysupplies a current to the semiconductor laser LD, and a bias currentsetting terminal 14 may be implemented so that the bias current valuemay be externally set.

FIG. 15 is a block diagram illustrating a relation between the ASIC 30realizing the semiconductor laser drive circuit (LD driver), the CPU 31that administers overall control of the semiconductor laser driveapparatus, and the image input apparatus 32. A temperature sensor fordetecting the temperature of the semiconductor laser LD is implemented,and the initialization is arranged to be preformed according to thedetected temperature of the semiconductor laser LD. In other words,since the characteristics of the semiconductor laser LD change accordingto a change in temperature, the initialization is performed according tothe temperature change so that inconveniences pertaining to offset lightemission due to a change in the differential quantum efficiency η fromthe semiconductor temperature change can be avoided. It is noted thatthe environmental temperature change can be estimated to a certainextent even without implementing the temperature sensor 15, and thus,the initialization may alternatively be performed at predetermined timeintervals, or after printing a predetermined number of copies or dots atwhich point a temperature change is expected.

The CPU 31 inputs the initialization command signal to the ASIC 30, andthe image input apparatus 32 inputs the light emission command signal,the delay control signal, the threshold current ON signal, the samplehold signal, the light emission control signal, the bias current settingsignal and the Isub setting signal.

In the following, the initialization process and the functions of theASIC according to the present embodiment are described in greaterdetail.

(1) After the power is turned on, and before the first semiconductorpower control (APC) operation, a threshold current Ith and a lightemission current Iη for a LD unit that is connected to the ASIC 30 aredetected. In the following descriptions, this detection process isreferred to as initialization.

(2) The threshold current Ith is set at the sample hold circuit 3, andafter initialization this current is continually corrected by APC. Thelight emission current Iη is set to the code of the D/A unit 10, and thecurrent value of this current is fixed until the next initialization.

(3) After initialization, a bias current of approximately 1 mA isconstantly supplied regardless of light on/off status.

(4) The threshold current Ith is generated by adding a set current Ishand a bias current Ibi at the sample hold circuit 3.i.e., Ith=Ibi+IshThe light emission current Iη is generated by a DAC current Idac that isoutput from the D/A unit (DAC) 10.i.e., Iη=IdacAlso, the LD drive current Iop can be obtained as follows.

$\begin{matrix}{{Iop} = {{Ith} + {I\;\eta}}} \\{= {{Ibi} + {Ish} + {Idac}}}\end{matrix}$

(5) The modulation signal realizes on/off control. The modulation signalcorresponds to LVDS (low voltage differential signal) format data,namely, DATA and DATA B signals in FIGS.16 and 17. Thus, the on/offcontrol is realized by the DATA and DATA B signals.

(6) The threshold current Ith is generated approximately 8 ns before thesemiconductor laser LD is switched on, and during the Ith generationtime period, the semiconductor laser LD is activated so that a lightemission pulse width that is equal to the DATA pulse width can beobtained. Then, during the semiconductor laser LD ON period, the lightemission current Iη is added so that an appropriate amount of light maybe obtained.

(7) After a light-off signal is received and a halt in the supply of thelight emission signal Iη is detected, the supplying of the set currentIsh may be stopped (see timing chart of FIG. 16). Alternatively, in acase where the light-off period is short and the next set current Ishhas to be supplied before the set current Ish can be stopped, the setcurrent Ish may be continually supplied (see timing chart of FIG. 17).

(8) When temperature change occurs upon completion of the initializationprocess, and in a case where the threshold current Ith changesaccordingly but the light emission current Iη is maintained at a fixedvalue, the threshold current Ith is compensated for as is shown in FIG.18 (Ith1 and Ith2).

(9) In a case where the differential efficiency η of the semiconductorlaser LD decreases with an increase in the temperature, when thetemperature of the semiconductor laser LD shifts to a higher temperatureafter the initialization, the light emission current Iη for obtainingthe same amount of light increases. Since the LD drive current Iop isset to an appropriate value (increased value) by APC whereas the lightemission current Iη has a fixed value set by the D/A unit 10 atinitialization, the difference (deficit) is compensated for by changing(increasing) the set current Ish. As a result, the current suppliedduring the Ith generation period may be greater than the actualthreshold current Ith so that the semiconductor laser LD may emit light(see FIG. 19).i.e., Ibi+Ish>IthThis condition is maintained until a re-initialization process isperformed.

(10) As a countermeasure for the above problem of the semiconductorlaser LD emitting light during the Ith generation period when thetemperature increases after initialization, an arbitrary current Isub isadded to the detected light emission current Iη to set a DAC currentIdac.i.e., Idac=Iη+IsubThe arbitrary current Isub is arranged to have a value that is greaterthan the difference between the light emission current Iη N at the timeof initialization, and the light emission current IηH when thetemperature is raised.i.e., Isub>IηH−IηN

Herein, the drive current Iop of the semiconductor laser LD may berepresented as follows.

$\begin{matrix}{{Iop} = {{Ith} + {I\;\eta}}} \\{= {\left( {{Ith} - {Isub}} \right) + \left( {{I\;\eta} + {Isub}} \right)}} \\{= {{Ibi} + {Ish} + {Idac}}}\end{matrix}$The characteristics described above are illustrated in FIGS. 20 and 21.

(11) The Ith generation timing may be arbitrarily controlled from anexternal terminal as is shown in the timing chart of FIG. 22. That is,the threshold current Ith may start being generated 8 ns or more beforethe semiconductor laser LD is switched on so that fluctuations in theamount of light due to thermal characteristics of the semiconductorlaser LD may be reduced. When the Ith generation period is lengthened,and the influence of the offset light emission of the semiconductorlaser LD on the image being formed becomes apparent, the arbitrarycurrent Isub may be increased so that the current value of the currentbeing supplied during the Ith generation period may be decreased. Inthis way, influences from offset light emission can be avoided.

As can be appreciated from the above descriptions, according to thepresent embodiment, the following and other advantages may be realized.

{circle around (1)} During non-emission time, no current is supplied, orotherwise, a fixed bias current is supplied, and right before the lightemission time, a current that is close to but less than the lightemission threshold current is supplied. Thus, an optimal laser emissionpulse that can prevent inherent light emission when light emission isunnecessary, and prevent turn on delay at the light emission time, canbe obtained.{circle around (2)} Since an operation for determining the lightemission threshold current is arranged to be performed at the time thepower is turned on or when a job is to be started, unnecessary orundesired light emission for determining a differential quantumefficiency can be minimized, and unnecessary or undesired forming oftoner image on the photoconductor and degradation of the photoconductormay be prevented.{circle around (3)} Since an operation for obtaining the differentialquantum efficiency is arranged to be performed based on a current forobtaining a predetermined amount of light, and a current for obtaining aportion of the predetermined amount of light, the light emissionthreshold value may be obtained in a practical manner.{circle around (4)} Since the difference between the light emissionthreshold current and the current that is close to but less than thelight emission threshold current may be arbitrarily set, the amount ofinherent light emission from supplying the current that is close to butless than the light emission threshold current right before the lightemission time can be controlled.{circle around (5)} Since the difference between the light emissionthreshold current and the current that is close to but less than thelight emission threshold current may be arbitrarily set, the amount ofinherent light emission can be prevented from becoming unnecessarilylarge even when the difference between the threshold current and theoperation current changes due to a change in the differential quantumefficiency in response to a change in environmental temperature.{circle around (6)} Since the difference between the light emissionthreshold current and the current that is close to but less than thelight emission threshold current is arranged to be no more than severalmA, and furthermore, since the supply time of the current that is closeto but less than the light emission threshold current may be arbitrarilyset, and this current may be freely supplied at any time, the lightemission delay (turn on delay) with respect to a light emission time canbe controlled, and the amount of inherent light emission from thecurrent that is close to but less than the light emission thresholdcurrent may also be controlled.{circle around (7)} Since an operation of controlling the current thatis close to but less than the light emission threshold current isperformed by monitoring the temperature of the semiconductor laser anddetermining the differential quantum efficiency according to an increasein the temperature, the amount of inherent light emission can beprevented from becoming unnecessarily large even when the environmentaltemperature within the apparatus is changed.{circle around (8)} Since an operation of controlling the current notreaching the light emission threshold current is performed bydetermining the differential quantum efficiency at predetermined timeintervals, the amount of inherent light emission can be prevented frombecoming unnecessarily large even when means for monitoringenvironmental temperature change is not provided.{circle around (9)} The above described advantages may be realized in awrite apparatus implementing a polygon mirror and an imaging apparatusimplementing such a write apparatus.

The present application is based on and claims the benefit of theearlier filing date of Japanese Patent No. 2003-010205 filed on Jan. 17,2003, the entire contents of which are hereby incorporated by reference.

1. A semiconductor laser drive apparatus comprising: a modulation signalsupply unit configured to supply a modulation signal to a semiconductorlaser, the modulation signal controlling a drive operation of thesemiconductor laser; a bias current supply unit that is arrangedparallel to the modulation signal supply unit and configured to supply afixed bias current to the semiconductor laser; and a control currentsupply unit that is arranged parallel to the modulation signal supplyunit and configured to supply a control current to the semiconductorlaser based on a switch operation controlled by a threshold-on signal,an amount of the control current controlled by a sample hold circuitwhich is controlled by a sample-hold signal independent from themodulation signal and samples a light emission threshold current of thesemiconductor laser.
 2. The semiconductor laser drive apparatusaccording to claim 1, wherein the sample hold circuit is controlled by asample-hold signal inputted from an external of the semiconductor laserdrive apparatus.